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Show 112 CRUSTACEA, I ~~ • J SECOND GENERAL DIVISION. rr, ENTOMOSTRACA. Under this denomination, which is taken from the Greek and signifies Insects with shells, Othon Frederick Muller comprises the genus Monoculus of Linnreus, to which we must add some of his Lernrere. His investigation of these animals, the study of which is so much the more difficult as they are mostly microscopic, and the observations of Schreffer and of M. Jurine, Sen., have excited the admiration and secured the gratitude of every naturalist. Other but partial labours such as those of Randohr, Straus, Herman, Jun., Ju. rine, Jun., A. Brongniart, Victor Audouin, and Milne Ed· wards, have extended our knowledge of these animals and · particularly of their anatomy ; but in this respect, Straus, as well as M. Jurine, Sen., although preceded by Randohr in the observation of several important details of organization, of whose memoir on the Monoculi, 1805, they seem to have been ignorant, has surpassed them all. Fabricius merely adopted the genus Limulus of Muller, which he placed in his class of the Kleistagnatha, or our family Brachyura of the order De· capoda. All the other Entomostraca are united as by Lin· nreus in one single genus, Monoculus, which he places in h~ ,e.Jass of the Polygonata or our Isopoda. These animals are all aquatic and mostly inhabit fresh wa· ter. Their feet, the number of which varies, and that some· times extends to beyond a hundred, are usually fitted for natation only, being sometimes ramified or divided, and some· times furnished with pinnulre or formed of lamellre. Their brain is formed of one or two globules. The heart has always the figure of a long vessel. The branchire, composed of hairs or setre, singly or united, in the form of barbs, combs or tufts, ENTO~lOST HACA. 113 ti'tute a part of those feet or of a certain number of them, cons . h · · d etl'mes of the upper mandibles(!). Hence t e or1gm an som . . h of our term Branchiopoda, affixed to these animals, of wh1c first we formed but a single order. Nearly all of them d . are provided with a shell com posed of one or two p1eces, very thin, and most generally almost me~branous .and nearly diaphanous, or at least wi.th a large anterio.r thoracic segment, frequently confounded w1th the head, whiCh appears to re-lace the shell. The teguments are usually rather horny phan calcareous, thereby approximating these animals to the ~nsecta and Arachnides. In those which are provided with ordinary jaws, the inferior or exterior are always exposed, all the foot-jaws performing the office of feet properly so called, and none of them being laid upon the mouth. The second jaws, those of the Phyllopa a: most .ex.cept~d, resemble these latter organs ; J urine sometimes d1st1ngmshes them by the name of hands. These characters distinguish the gnawing Entcimostraca from the Malacostraca; the others, those which constitute our order of the Pmcilopoda, cannot be confounded with the Malacostraca, inasmuch as they are deprived of organs of mastication, or because the parts which seem to act as jaws are not united anteriorly nor preceded by a labrum as in the antecedent Crustacea and the gnawing Insecta, but are simply formed by the branches of the locomotive organs, which, for that purpose, are furnished with small spines. The Pcecilopoda in this class of animals represent those which in that of Insects are kno"vn by the name of Suctoria or the Suckers. Nearly all of them are parasitical, and they seem to lead to the Lernrere by insensible gradations ; but the presence of eyes, the faculty of changing their skin, or even of undergoing a sort of metamorphosis(2), and that of locomotion by (1) See Cypris. (2) The young of Daph~a, and of some neighbouring subgenera, and probauly also those of Cypris and Cytherea, with the exception of size, scarcely differ, if at aU, from their parents on quitting the egg; but those of Cyclops, the Phyllopa, and the Arguli, experience considerable changes while young, either as respects VoL. III.-P |
